Another option is to buy a license of Sambar, while not the most capable SMTP and not the best IMAP server, it is exceedingly reliable. I would suggest that you consider trying Surge, Steffen would agree, and I beleive you can test that for no charge.

But if you decided on using Sambar you can run the web side of it for some things on a different port, then use Apache to reverse proxy into it. More over, Sambar offers a very nice Chron task service, which I use for somethings. Also, you can set up SSL with Sambar far easier for such things as remote mail server management securely via HTTPS.

I admit fully that Sambar is not the best mail server, that being said, it is very easy to learn and currently it handls about 600 to 700 busy email accounts for me right now...and when I say busy, I mean some of these accounts are very busy. The Sambar SMTP has never crashed on me, not one time.

Then in your PHP scripts and via the PHP_INI you can configure it to communicate with your mail server very easily. Another option may be to relay through your current ISP, generally (and wisely) they may require authentication (user/pass) to relay email, which should be easy enough to set up.